This invention relates to capacitance membrane switchcores that may be used with a keyboard for data entry apparatus, such as a computer terminal, word processor, etc., of the type comprising appropriate conductive circuits carried on flexible plastic film membranes.
An electronic keyboard is an essential user interface device required for the input of information for many types of data processing systems. The principal elements of the keyboard comprise keys supported for actuation by an operator, an array of switches to develop an electrical signal in response to actuation of the keys, and electronic circuitry associated with the switch array for detecting the actuation of an individual key. The associated circuitry typically includes drive means for scanning the switch array at a high rate and sense means for detecting the change in an electric characteristic upon closure of a specific switch by manual actuation of the key.
The switch array for a state of the art keyboard is now generally a membrane switchcore comprising a laminated structure of two thin flexible plastic films, each carrying selected circuitry applied thereto by printing or vacuum deposition techniques. Flexible plastic film membrane switchcores of this type are to a large extent replacing circuit boards of hard rigid plastic such as phenolic or epoxied fiberglass on which the circuits are formed by metal plating and etching methods, as had been in common use prior to the development of flexible membrane elements. The membrane switch array may be either of the contact type or of the capacitance type. In a contact switch array, one set of contacts comprising a switch is connected across each crosspoint in the array. As is known in the art, a contact switch is easily decoded with direct current (DC) drive and sense signals, but cannot be accurately decoded if more than two keys are depressed simultaneously. The latter limitation is referred to as "2-key rollover", and is a result of phantom paths created with multiple key depression.
Capacitance type switch arrays include one or more capacitors at each crosspoint in the array and operate by presenting a change in the capacitance value at a crosspoint in response to actuation of the associated key. Upper and lower conductive circuits are spaced from and separated from one another by a dielectric layer, and are used to interconnect the various capacitor plates to drive and sense lines. The drive and sense circuitry means connected to the capacitance membrane switchcore are designed to charge each key cell capacitor by an alternating, or pulse current source, ascertain the net capacitance by known techniques at a key cell, and determine whether the capacitance value is high or low relative to a reference value, so as to thereby sense key actuation. The means to achieve a change in capacitance across a crosspoint may comprise either a variable capacitor or a combination of a fixed capacitor in series with a conventional contact switch.
Variable capacitance type switch arrays can be characterized as including at least one moving plate, in which the change in capacitance is achieved by physically moving a capacitor plate at a crosspoint; this is usually accomplished by utilizing the actuation force to deflect a capacitor plate located on a flexible membrane or on a key. In other types of variable capacitance switch arrays, e.g. a rigid, movable plate is linked to the key actuating means to define a variable capacitor which may be electrically in series with a second fixed capacitor at each crosspoint. The second, fixed capacitor may also be used in conjunction with a variable capacitor membrane switch array. The other type of capacitance switch array (e.g. fixed capacitor/contact switch) comprises a fixed capacitance in series with a conventional contact switch across each crosspoint in the array. The switch is closed when a key is actuated and the associated fixed capacitor is thereby connected across the crosspoint; the fixed capacitor is effectively removed and the crosspoint open when the key is released. Capacitance switch arrays can also be classified as "open" or "closed" depending upon whether one of the circuits, usually the upper circuit of the array, is open to the environment or closed from the environment.
Capacitance membrane switchcores exhibit numerous operational advantages as compared to contact membrane elements. Capacitance arrays are preferred in applications where multiple key rollover is desired because the phantom paths can be suppressed by known decoding techniques. Another advantage of capacitance arrays is that since alternating current (AC) signals are used in decoding the capacitive elements, a complete conductive path is not required, which provides greater reliability and longer life as compared to contact switch arrays.
However, capacitance membrane switchcores present several difficulties, in part resulting from the spatial relationship of two separate circuits, which have inhibited their widespread use in keyboards. One of these is the requirement to supply drive signals to one of the conductive circuits of the switchcore and sense capacitance changes from the other conductive circuit; this causes several problems since the two conductive circuits are located along opposite surfaces of a dielectric layer. Another is that the open capacitance membrane switchcores of the type now known in the art are more expensive to manufacture than contact membrane switchcores due to the additional processing steps required by the nature of their structure. Specifically, trace layout is difficult or impossible to accomplish without a means for establishing connections between trace layers. In prior open capacitance switchcores, such connections required die cutting of apertures through the insulating layers to establish physical contact between trace layers. This problem is prevalent in both open and closed capacitance membrane switchcores, as well as contact membrane switchcores. Further, the routing of conductive elements of circuits of a capacitance membrane switchcore can present a designer with problems not found in contact membrane switchcores for example, the effects of stray capacitance. The prior art techniques for resolving these and other problems associated with the design and manufacture of capacitance membrane switchcores have not resulted in optimum utilization of their desirable characteristics; hence, there is a need for further advancement in the development of capacitance membrane switchcores and this invention was conceived as a fresh approach in this art.
My present invention provides a capacitance membrane switchcore of the type having first and second conductive circuits along opposite surfaces of a dielectric layer. Each circuit includes a plurality of conductive traces and a plurality of capacitor plates connected to each trace to define a switch matrix including a fixed capacitor having a capacitor plate from each circuit that is unique to each key cell of the matrix. In one of its aspects, my invention provides a conductive third circuit including capacitor plates and traces that is physically independent of the first and second circuits but located along the same surface of the dielectric layer as one of said circuits wherein the third circuit includes capacitor plates aligned with capacitor plates of one of the other circuits in such manner as to provide intertrace capacitive coupling therewith; conductive traces of the third circuit extend from its capacitor plates for connection to external electronic circuitry. This feature of the invention provides a capacitance membrane switchcore in which conductive traces that are to be connected to drive and sense circuits are all located along the same surface of the dielectric layer. In another aspect, my invention is concerned with a construction for conductive traces of a circuit of the switchcore wherein a trace may extend partway along a first surface of the dielectric layer, have one or more branches located along the opposite second surface of the dielectric layer, and then return to the first surface of the dielectric layer, wherein the several branches of the trace are coupled to one another by means of intratrace capacitive coupling.